text:
Does the Prandtl flying wing have a lower maximum speed than other designs?
Does the design of the wing with its "twist" near the wingtips necessarily limit the maximum speed of an aircraft utilising that technology?
Also, with the leading edge profile/attack angle varying along the length of the wing, does this generate any unusual strain requiring complex engineering to accommodate?
( I am a novice here-as I am sure you can guess but a very determined one and I am going to build a Micro Turbine-powered(hopefully!) Prandtl-type wing for myself this year, I may need to consult the many experts here on a regular basis.)
Please try to minimise any mathematical equations in your answers since I’m a bit rusty on my math.

Related: https://aviation.stackexchange.com/q/57662/23223

"Please try to minimise any mathematical equations in your answers" trying to understand aerodynamics without math is like trying to drive a nail without a hammer, possible, but prone to errors and head injury.

@gregor kropotkin if it is high speed performance you are after G loads will neccesitate a lower aspect design, rather than the graceful Prantyl wing.  A twist in a swept wing is generally beneficial. Proverse yaw can cleverly be created by lowering a leading edge, rather than raising a trailing edge as with standard ailerons. On the opposite side, raising the leading edge creates less drag, and you get a roll and yaw in the same direction.  Spoilers on the inside wing are employed in a similar manner.

Short answer: Yes.
Maximum speed is correlated with wing loading. A high wing loading shifts all speeds up. A flying wing will always have a lower wing loading in comparison to a conventional design when payload, range and landing speed of both are identical. So you will start at a disadvantage.
Due to the high lever arm of the conventional tail, the conventional configuration can use high-lift devices (or highly cambered airfoils) for a much higher maximum lift coefficient than what is possible in regular low-pitching-moment airfoils. This allows to keep the wing small for the same landing speed. Unfortunately, the flying wing is restricted to those low camber or even reflex airfoils because its surfaces for creating a pitch balance have a small lever arm and, consequently, a poor moment-to-lift ratio.
The design of the Prandtl wing leads to a bit of downforce at the tips when in normal flight in order to lower the wing's root bending moment. But only a bit. Sweep places this downforce in the aft region of the wing so the inner, more forward region has to create all the lift for carrying its own weight, the payload and, of course, compensate for that downforce, too. Since the low wing bending moment will allow for a light wing, the total lift requirement is rather low, too, but still encumbered by the aforementioned loads.
Now to the maximum speed: A flying wing will allow for very low drag coefficients. There are no intersections between parts and no tailboom which adds friction drag, so for a given wing loading a flying wing will allow to reach high speeds with limited power. However, the lower wing area of the conventional design with the same landing speed has less wetted area and so can easily accommodate that tailboom drag. Once the details like part intersections are well designed, the conventional design should come out ahead.
What you need to watch out at high speed is flutter: That wide, light, swept wing will start to flutter much earlier (at lower speeds) than the rigid, strong, small, straight conventional wing. With sweepback you will encounter a speciality of flying wings, a coupling between the short period mode and the wing bending eigenfrequency. Since the short period mode frequency goes up with speed and the bending eigenfrequency is constant over speed, flying fast will bring both together and require a stiff wing in order to shift that point up. Don't forget that pitch damping of flying wings is an order of magnitude lower than that of conventional designs, so the short period mode will show up in a Prandtl wing if you go fast enough.

Horten gliders.

"The idea of a Prandtl wing is to create a bit of downforce at the tips" Is that right? Prandtl's original analysis described a bell-shaped curve with positive lift approaching zero towards the tips. Yes the airfoil there has negative incidence, but is cambered enough to actually produce a small lift-and-thrust component.

@GuyInchbald: Prandtl's lift distribution was for the maximum bending moment; regular flight would be at a much lower lift and angle of attack. With the low lift near the tips, this reduced angle of attack means a downforce there in level flight.

@PeterKämpf OK, thank you. I guess something like "The design of the Prandtl wing leads to a bit of downforce at the tips when in normal flight" would be clearer to me.

Twisting the wing tips to a lower angle of attack helps create an "elliptical" lift distribution,  which reduces stress loads on the wing the farthest away from the wing root.
However, if one ever watches the take-off roll of the Rutan Voyager, one can see the dangers of this design combined with a tricycle landing gear.  In this (extreme) case, the wing twist caused the wing tips to have a negative angle of attack, causing the wing tips to bend downward and scrape the ground until the nose was lifted.
It is here that retractable slats are most beautiful.  At higher angles of attack, dropping slats increases the "twist" of the wing, greatly improving stall characteristics.  At lower angles of attack, such as in cruising, they can be retracted to reduce drag.
The Voyager was an extreme case where a slightly longer nose strut may have helped.
For an R/C model powered by a microturbine, or electric motor, some twist or "downwash" on the wingtips will be very helpful.  You may find slower flying this wing with less power, or even gliding it, to be more enjoyable.
At higher speeds, all potential sources of wing loading, including G forces and flutter, need be carefully evaluated for safety.

As an aside, I've seen first-hand evidence of down-lifting wingtips in my own flying.  When cruising at high airspeed, the lower side wires of a hang glider can actually be slack.  Although the net lift contribution of each wing obviously has to be upward, if we view the wings as hinged at their roots, the net torque contributed by each wing about this hingeline can actually act in the downward direction.  A solid wing strut would experience a compressive load in this situation; in the hang glider case the download is borne by the upper side wires, running from the wings up to the kingpost.

For added clarity re the above comment, bear in mind that many hang glider wings are highly tapered (essentially a modified delta shape).  Most of the lifting area is concentrated well inboard, but the small amount of lifting area (or at high speed/ low angle-of-attack, downlifting area) that is located near the wingtip can contribute lots torque by virtue of acting at a long moment-arm, as measured to the aircraft centerline ("keel tube").

"Twisting the wing tips to a lower angle of attack helps create an "elliptical" lift distribution, which reduces stress loads on the wing the farthest away from the wing root." You're misunderstanding OP, they are talking about the twisted flying wing, like in this question

@AEhere supports Monica There are certainly many ways to design a wing.  "Twist or washout" will reduce the lift coefficient of that section of the wing, giving a stronger rectangular wing an "elliptical" lift distribution.  And by the way, are not all wings designed to fly?  You can attach a tail and fuselage if you'd like, and get something like ... a DC-3!.  Thanks for your comment.

Robert, the idea of a Prandtl wing is to not use an elliptical distribution. The outer, rear wing actually creates a downforce, like the bell lift distribution of the Horten flying wings. Sorry, no upvote this time.

@Peter Kampf. Your point of view is much more appreciated.  Of course, what force any part of the wing creates will depend on AOA.  Very much interested with what goes on with airflow going from positive to 0 to negative local AOA, especially with swept wings.

Your question has been at issue ever since the early pioneer days of flight. British pioneer J.W. Dunne discovered the principle around 1905-6 and developed a tailless swept monoplane with drooped and washed-out wingtips. He used their negative lift to provide lateral as well as longitudinal stability. He patented it in 1909 (UK pat. 08118), got it flying as the D.7 in 1911, and lectured the Aeronautical Society on its aerodynamics in 1913 ("The Theory of the Dunne Aeroplane"). His rivals and the military bureaucracy were all convinced that those wing tips must be horribly inefficient, despite its evident firecracker performance and the reports of the serving officers who encountered it. They became so persuasive that at one time poor Dunne even began to believe them himself. So I am intrigued to see this hoary old debate rising from its coffin yet again after more than a century has gone by.
Ultimately, the wingtips combine two functions, as the tail stabilizer split in half and stuck on the ends of the wings, and as vortex-reducing winglets. Compared to a conventional layout they lack the drag of a tail fin and have one less set of tip vortices when the horizontal stabilizer has work to do. Moreover they allow the wing structure to be lightened, Prandtl's key discovery when he re-examined them in the 1930s. Does all this counter the disadvantages of a larger than normal span and a stabilizer moved further forward where it is less effective? Should the evidence of the D.7 be taken at face value or does it really hold the plane back? Its proponents publish analyses which say yes, its critics raise snags which say no.
We may recall Convair's F-102, F-106 and B-58, all of which featured tailless wings with progressive conical washout after the manner of Dunne (though independently rediscovered) and were renowned supersonic "hot ships". The conical droop did add some drag but not a lot. What penalties would an alternative solution to the problems it solved have brought? Would that have made the planes faster or slower?
Frankly, the world has no better idea in 2020 than it did in 1911.

Very well written answer.  The world does have a better idea indeed 2020 in airliner design (actually when they built the DC-3).  Study polars and see how Lift/Drag ratio suffers in cruise when wing is not at optimal AOA.  This is why the variable AOA wing Dunne so brilliantly conceived at the dawn of aviation was superseded by movable slats.  And deltas do have additional drag penalties of their own.  However, some washout in a model or even a recreational aircraft where fuel efficiency is not critical is a great idea.

Uh - conical camber is no washout. Not at all! It uses camber on a supersonic airplane (albeit with a subsonic leading edge) to eke out a bit of suction with a forward tilt. And Dunne was not alone - Igo Etrich and Franz Wels flew a glider with a similar stabilisation technique as early as 1907.

These comments are excellent examples of how the nay-sayers have updated their criticisms over the years, just as the advocates have refined their case. But there was a sharp contrast between the "seagull" aerodynamics of the Dunne and the "zanonia" or "crow" aerodynamics of Page and Weiss. Oh, and if you visit RAF Cosford you can check out the B.Ae EAP, a supersonic delta with washout that postdates the Convair designs by several decades.


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Did the Germans ever use captured Allied aircraft against the Allies?
I have noticed that during WWII the Germans captured quite a few P-47s, P-51s, Spitfires, and other Allied aircraft. I know that they tested them extensively but wanted to know if they ever used them against Allied aircraft.

Googled "captured B-17", got https://en.wikipedia.org/wiki/Kampfgeschwader_200 , https://www.warhistoryonline.com/military-vehicle-news/captured-b-17-bombers-in-world-war-ii.html , https://www.historynet.com/japans-fleet-flying-forts.htm .  A bit more persistence would yield many more similar links, no doubt. Perhaps after a bit of looking you'll end up sharing a self-answer?

Not really an answer but there are quite a few reasons why this would not be a good idea. 1) Without training, manuals etc. you'd need to be quite an ace to get into an unfamiliar aircraft and fly it with any ability, 2) It's only one plane. Most tactical actions require squadrons to achieve anything worthwhile, 3) It would be very dangerous; in the heat of battle, the risk of getting shot down by your own side would be high. It might've been useful for a top secret covert operation - don't know if that ever happened...

2 does not really apply to French planes. The Germans must have captured 100s after the fall of France which they probably passed on to Vichy France.

And then there's maintenance.  Warplanes need parts, tools and procedures that are sometimes specific to that model.

Not sure about the Germans, but the Italians used a P-38 (from the site 12 O'Clock High):

The piece about the P-38 captured by the Regia Aeronautica is
extremely correct (at last, I was sick of reading accounts or
questions mutuated from the complete fantasy of Martin Caidin...! ),
except for one detail: the plane shot down on August 11, 1943 by Col.
Angelo Tondi with the P-38 wasn't a B-24 but a B-17 instead...

and later on the same post:

This is extracted from the diary of 2nd Lt. James Franklin Boston:
"August 11, 1943-Mission to Terni, Italy 06:00. Carried 6000 lbs.
demos. (demolition bombs). Flak moderate. Coming back 60 miles from
Italy intercepted by ME-109 and captured P-38 with enemy flier flying
it. Nobody fired at first, thinking it friendly. Saw it slip in a
formation from behind and shoot down a B-17. Everybody fired at it
then including myself but my gun jammed. 20mm shell came by my head
while I was looking through astrodome and went into No. 2 engine. P-38
was not shot down."

Yes they did.
Kampfgeschwader 200 flew recon and supply missions in captured bombers.
Here's a good few more captured examples: German Warplanes 1939-1945, Captured Allied Aircraft in Luftwaffe service Among them:

A Fairey Swordfish was maintained in serviceable condition for around six months, one has to ask why.
A special squadron, the Zirkus Rosarius, went around training front-line squadrons against them, not sure if you'd regard that as "operational", but it is more than mere test and evaluation.
Several examples are transports. Given the Kampfgeschwader 200 use of bombers in this role, it might seem unlikely that these others were never used in their intended role.

That's a nice collection of photos, but I don't see any references in there to actually using the captured front-line aircraft in active operations against Allied aircraft-- as opposed to for testing and evaluation.

@quietflyer Good point. I have modified my answer accordingly.

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How can I calculate the relationship between propeller pitch and thrust?
I am trying to design a WWI propeller so I started calculating. The propeller comes with a specified pitch of 1.75m. The engine is supposed to run at around 1900 rpm and 147kW. These are most likely not the maximums, but I use these values as a starting point.
So I calculated the propeller speed: 1900/60 * 1.75 * 3.6 = 199.5 km/h
I also found on the internet an angle of 16.88 degrees but the site mentions "angle of attack" which I deem incorrect. However if I treat the value as "effective pitch angle" I get about the same speed of ~195.68 km/h (using the formula from Koyovis for Φ from How to calculate variable pitch propeller parameters?).
Everything looks good, my reasoning at this point is that this prop is suitable to be used for the particular aircraft. The speed that I found seems to be reasonable for this aircraft.
But here comes the problem, this is the part that I don't understand and that is 'thrust': if the propeller reaches 1.75 pitch this means that the exit velocity - the input velocity (Ve - V0) is 0!
If I understand the formulas (such as here) correctly this means there can no longer be thrust. How would it possible then that the aircraft is still being pulled forward? This automatically means the aircraft has to slow down. But my assumption is that the propeller is 'designed' to go as fast as the specified pitch.
Can someone enlighten me please. Why does my propeller have to be slower than designed?

edit (I will leave my question above unaltered).
Clarification as per request of JZYL:
The propeller is: AB622
Some of these propellers can still be found on auction sites and such. They have markings like "G 1669 NO 90 DRG AB 662 CRH HISPANO SUIZA, WOLDSLEY VIPER D2400 M/M p 1750 2400mm"
The pitch is 1750mm
The diameter is 2400mm
The aircraft I am looking at is a WWI SE5a with a Wolseley W.4A Viper engine.
It is difficult to state exact parameters. There is a lot of 'belief' of what the aircraft at that time could do.
Some people say the aircraft top speed would be 138 mph (220 km/h). However might as well be one top speed ever recorderd for one plane at excellent flying contditions. Also some people claim the engine could run at 2200 RPM.
So instead I go for some 'safe' or nominal values:
Speed = 125 mph
RPM = 1900
The calculations that I make do not add up. The "pitch speed" (I don't know what the official term is) that I have calculated for the prop is 199.5 km/h (124.6 mph) with these parameters.
But if I have to take slip of say 10% into account (how much is normal, 5% 10%?).
In that case I go down to: 112 mph.
112 mph is way below what people see as the maximum speed of the craft.
So I am basically trying to find a clarification for this.

The geometric pitch ($p$) is simply the distance that a point on the prop would move forward in one rotation, if it's gripping in a solid medium. You can derive the pitch speed via this geometric relation if you wish, but I would caution against inferring performance from this number. Instead, you should consult propeller, engine and aircraft performance in their entirety.
Using your provided data, let's see if everything fits. We assume:

Prop RPM of 2200
Top cruise speed ($V$) of 125mph
Prop diameter ($D$) of 2.36m (from Wikipedia on SE5a)
Prop pitch ($p$) of 1.75m
Wing area ($S$) of 22.7 sq.m (from Wikipedia on SE5a)

From this reference, the reference pitch angle/blade angle ($\theta$) by convention is measured at 3/4 of the blade radius, and can be related to pitch via:
$$\theta=\tan^{-1}{\frac{4}{3} \frac{p}{\pi D}} \approx 17.5deg$$
The advance ratio is calculated, with $n$ being the prop rate of rotation in round per seconds:
$$J=\frac{V}{nD} \approx 0.65$$
Using the plot below as a rough reference, which is predicted for the McCauley 7557 propeller, gives a thrust coefficient ($C_T$) of 0.06.

Ref: http://webserver.dmt.upm.es/~isidoro/bk3/c17/Propellers.pdf
Now the thrust coefficient is defined as:
$$C_T=\frac{T}{\rho n^2 D^4}$$
where $rho$ is the air density and $T$ is the propeller thrust.
For level flight, thrust equals drag, and we can back calculate the drag coefficient:
$$C_D=\frac{2C_T n^2 D^4}{V^2S} \approx 0.07$$
This is high enough to be well within the realm of possibility for a biplane (if it's too low, then it's an indication that the performance numbers do not jive). For reference, Cessna 172 has a total drag coefficient of 0.032 at 122kt at SL. In conclusion, I would say that your numbers check out in the back-of-envelope sense.

I assumed no such thing. I clearly said that: "This automatically means the aircraft has to slow down". In other words: "The aircraft is slower". And this is what I am trying to find a clarification for. The aircraft speed then becomes much slower than what the books say.

@Veltro Then I have misunderstood your question. I suggest you modify your OP, cite what you believe the aircraft speed is, all the factual info, etc.

@Veltro Please see my modified answer.

Your CT looks a bit high (0.06 instead of 0.05). RPM of 2200 is too high compared to other sources I found (RPM of 2140 absolute max). Anyways I ran the formula's in my excel sheet. Eventually I used RPM 1945 D 2400 V 125 mph (Choosing the values carefully to get a Φ of ~16.88). J becomes 0.71. Lookup of CP 0.039 CT 0.046. Then calculated CD 0.04578. Compared to the CD found for the aircraft 0.0402 (zero lift) to 0.0440 (max speed) this value comes really close but still lower. So I guess the numbers 'jive'. Thanks! Now going to figure out how to calc the thrust.and if the engine matches.

@Сократ Geometric.

@Сократ What you've calculated is not geometric pitch at all; it's related to advance ratio.

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What could be the cause of an uncommanded roll at high speed?
I have a problem with a T-38. There is uncommanded roll to the left when the aircraft flies at 250 knots. But as the aircraft speed goes up around 500 knots, the uncommanded roll is now to the right. During 500 knots we could control the plane with 15-20 aileron trims.
What may be the problem?
We have replaced a lot of systems and cylinders. And we changed the travel limits of aileron, flap, rudder, and horizontal stabilizer. But no change has been observed.

It would probably help a lot if you indicated what aircraft you were referring to. Obviously, at 500 kt IAS, you're probably not talking about a Cessna 150, but there are a lot of aircraft that will hit 500kt.

Have you inspected the geometry of the wings?

You may wish to edit to clarify this -- as posted, it reads that you have left at low speed, then left at high speed.  If, as I suspect, they're opposite, this is different from the case where they're the same direction.

Have you checked the aileron actuating system and associated hydraulic lines?

Yes we checked all aileron system. The aircraft is    t-38

So both at 250 and at 500 knots the uncommanded roll is to the left?

No at 250 ,it is to left , at 500 to right.

Uncommanded rolls are opposite direction.

Is this an actual T-38, or a simulator?

What aircraft please? roll could be caused of course by ailerons, but also by spoilers even by non symmetrical elevators etc, but the fact that it changes side by increasing the speed, means that you have at least two sources of troubles that compensate each other partially as a function of speed except at a crossover speed somewhere between 250 kIAS and 500 KIAS

I once went on-site to troubleshoot an airliner with an uncommanded roll problem.  The operator had rigged this and rigged that and were in the process of ordering new aileron power control units, pretty much out of desperation.  I asked about the trim actuator, which they had replaced with a known good unit off another aircraft, making them think that couldn't be the problem.  I centered the trim and went to look at the position of the rig pin holes in the aileron control circuit.  They were mis-aligned.
The trim actuator on this airplane was an electric linear type with an internal Linear Variable Displacement Transducer (LVDT), that supplied the position signal to the cockpit trim indicator (or EICAS in this case), that was "dithering", that is, the signal was drifting around.  When you thought the trim was centered, it was actually offset, and this error would randomly change as the actuator was moved.  On the original airplane the actuator was installed on, the dithering wasn't enough to cause a snag to be raised by the flight crew so its internal problem went undetected.
The variations in roll you saw at speed may be some other phenomenon, or it just could have been coincidental variations in the trim indication on that flight.
So, if it was me, the trim system itself would be an item to cross off the list before proceeding to more desperate measures. Do a thorough functional test of the trim system and its indication, and also look for things like backlash within the actuator itself or in the linkage.

It sounds like roll trim is a function of speed, not a random dither.

Maybe, maybe not.  You would have to go out and do a carefully documented series of tests to establish what is going on precisely.  Was the shift with speed a direct, repeatable condition or just a one time coincidence.  We don't have all the information and are just spitballing here.  Golden rule of troubleshooting: eliminate the easy/cheap things first.

If this were a model aircraft with a wide speed range (like, say, a hand launch glider), I'd expect this to be a cross-trim problem.  Generally, the rudder will have more authority at lower speeds, while aileron will gain authority as speed increases.
Based on that, I'd suggest verifying the rigging of the airframe; look for a fin, rudder, or rudder trim that's generating yaw in the direction of your lower speed uncommanded roll (yaw will produce roll due to whatever mechanism provides roll stability).  The aileron trim that offsets that uncommanded yaw will overpower it as speed increases, producing your uncommanded roll in the opposite direction.

Good answer but if it were a model, I'd suspect that something was broken or loose and moving under the increased aerodynamic load of high speed. Or an incorrect thrust line.

@RobinBennett A thrust line problem is tied to power setting; it'll happen during takeoff/launch even more than at high speed (because less aero forces on the surfaces at low speed and high power).  Broken or loose will be erratic.

If the flight control is a hydraulic system, have you changed the fluid recently, if yes you need to purge the system for air bubbles, otherwise you need to check the fluid quality for impurity.

http://www.aiaahouston.org/Horizons/ATS2019-Presentation-SS-Tang.pdf
This presentation was given in AIAA Houston Section  Annual Technical Symposium (ATS) at NASA Johnson Space Center, Gilruth Center. A new discovered root cause for uncommanded roll, pitch and yaw was given.
A technical paper was presented and a live demonstration was shown in the 2019 AIAA AVIATION FORUM and EXPOSITION. A Youtube video shows this phenomenon.

Hope this may help you to understand the uncommanded roll. Actually, it was not "uncommanded". It was indirectly commanded.


text:
Why would a fighter drop fuel into a drone?
According to the BBC:

A feed from a camera fitted under the fuselage of the surveillance drone shows a Russian SU-27 making two extremely close passes while releasing, what appears to be, fuel as it approaches

Looking at the video it seems that the fighters are indeed releasing some liquid when passing close to the drone. It looks to me like with the speed of the drone any fuel dropped into the fuselage will quickly evaporate.
Would the drone be in risk of catching fire when getting in contact with the fuel? Would that be a plausible reason for releasing fuel into it?

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Maybe they thought that the drone was running low of fuel and were trying to be helpful

See this and other answers to a related question -- https://aviation.stackexchange.com/a/47997/34686

I read about four possibilities which the SU-27 pilots wanted to enforce:
a) Covering the drone in fuel and then igniting it with the afterburners which should have lead to a fire and subsequent malfunction of the drone
b) Generating a strong wing-vortex and therefore severe turbulence, which should have led to the crash of the drone. For similar reasons, airliners are required to keep a minimal separation. The smoke which appears in the video would then have to be explained by the formation of a contrail which suddenly forms because the pilot added power just when passing the drone. This answer is unlikely, as the smoke was identified to not being smoke from a contrail.
c) (Edited after comment of 757toga) Injecting jet-fuel into the turboprop engine of the reaper, leading to starvation from ingesting fuel-rich air.
d) The SU-27 pilots wanted to blind the optics of the drone by applying jet-A1 fuel.
In the end, the SU-27 pilots chose option e), bending the motor shaft and propeller of the drone by virtue of a direct collision. I do not believe that the fuel of the SU-27 is enough to bend exactly one propeller, therefore I believe a collision occured.
It should be noticed that the exact intentions of the pilots is of course unknown, I also do not believe that they will tell anyone, but they succeded.

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@757toga corrected it, thought I already had...

See this and other answers to a related question -- https://aviation.stackexchange.com/a/47997/34686

I don't know about catching fire, although they might get lucky, but I'd imagine that dumping a hefty quantity of liquid into the engine intake might well upset the engine enough to turn the drone into a glider, before it has a chance to evaporate. Possibly it would also damage or impair the sensor package.

Yes, but as it clears out the engine should restart--long before it hits the sea.

@LorenPechtel doesn't have to. If the MQ-9 spins out of control, aerodynamic forces may well rip it apart during its fall for example. And if it's pretty low (quite possible, MQ-9 isn't Global Hawk but a tactical asset) it may simply have run out of altitude before it could restart.

I think the "fuel dump" is a complete secondary issue.
Whether it was a normal fuel dump, or unlit fuel poured into the unlit afterburner stream, or AB, or wake turbulence....
It was a purposeful move on the part of the Su-27 pilot to harass and disrupt the flight of the MQ-9.
The first pass did not give him the desired result. On the second pass, he misjudged and went a meter or so too close. Resulting in impact with the prop and probably tailplane.
And it can't be said that the Reaper just magically appeared in front of him with no time to maneuver, due to the vastly different speeds. He came around and did it again. And any competent Air Force would know the existence of the Reaper (and all the other aircraft in the area). "Hey, there is traffic at your 11 o clock, at 14k feet. Don't hit it."
The Reaper operators knew they was there, due to the camera aim. And then changing the camera angle for a better picture on the second pass. But not much they could do to avoid.

Wouldn't it have been far easier (and safer for the pilot) to just shoot it down?  I mean, yeah, it'd be seen as an act of aggression and all that, but so was what they actually did, so I don't think that'd make any difference...

@DarrelHoffman - Absolutely easier. But also, an overt act of aggression. Russian fighter jet shooting down a US aircraft, over international waters? That is a LARGE escalation of the status quo.

Making two close passes to disrupt a drone is also an act of aggression.  Is the argument here that the intent might not have been the same as the end result, of fully downing the drone?  Agreed you couldn't make that argument if guns had been used.  These close passes leave open the interpretation of it being an attempt to temporarily disrupt, downing being an accident (@DarrelHoffman). Perhaps that's why US generals have called it "reckless" flying by the Russians, rather than act of aggression, because they don't want an escalation, and their public reaction is part of determining that?

@PeterCordes Maybe it's just a matter of plausible deniability. IF the Russian government, or high ranks, did order to disable the drone, a direct gun hit would be undeniable. Using the "fuel dump attack" could be afterwards denied or justified by saying it was a reckless maneuver by the pilot, an initiative of the pilot himself to harass the adversaries (fighter jet pilots are not unknown of such jackassery, after all) or something like that.

@PeterCordes It could also be the truth: IDK the actual level of professionalism and competence of the average Russian jet fighter pilot, but given how the war in Ukraine is going for Russian troops, I could well see a (maybe inexpert), frustrated and pissed off pilot trying to "ruffle the feathers" of the drone, without knowing it could actually end this bad (the collision that could have bent the propeller blade could well be interpreted as a sign of inexperience of the pilot).

@LorenzoDonatisupportUkraine: Senator (and former fighter pilot and astronaut) Mark Kelly drew that conclusion, that the actual collision was incompetence on the part of Russian fighter pilots while attempting whatever they were doing with the fuel dump: https://www.businessinsider.com/mark-kelly-russian-jets-us-drone-incident-shows-pilot-incompetence-2023-3

@PeterCordes thanks for the pointer!

Several possibilities exist as well as nonsense theories.
Possible scenarios

The dumped fuel could cause the drone engine to fail.
The dumped fuel could cause some sensors to fail and indeed obscure the cameras

What is unlikely though, if at high altitude where the fuel is way below flashpoint, that it would ignite.
The next questions for which I do not have answers are

Does the Su-27 have fuel dump capabilities?
If so, why are the dump nozzles right near the engine exhausts as opposed to most airliners, the dump nozzles are near the wingtips?

But in the video, the drone did not pass through the dumped fuel trail.
Wake turbulence could have upset it as well.

"why are the dump nozzles right near the engine exhausts" – I can't tell you that, but it is not unique: The F-111 Lancer famously has its fuel dump nuzzles right between the engines and can light the dumped fuel with the afterburner. Totally useless, but a great airshow stunt. On the F-111, the location of the fuel dump nozzle is dictated by the fact that it is one of the first swing-wing fighters, and the designers apparently didn't want to put too much complicated stuff into the wings.

I don't understand the fuel bit at all.  I see little chance of appreciable damage.  Wake turbulence is another matter, though--from when I first saw this story I figured the pilot was trying to disrupt it that way.

Discussion on another answer (since moved to chat where the useful info is much less likely to be seen) suggests that the SU-27 can "dump" fuel through its afterburner fuel lines, apparently without igniting it.  https://www.metabunk.org/threads/debunked-military-chemtrails-on-off-su-30-smoke-skywriting.2926/#post-83910

@PeterCordes -- also addressed in this and other answers to a related question -- https://aviation.stackexchange.com/a/47997/34686

The idea of lighting the drone on fire is really more of a joke, just because it isn't a feasible method, as you said, it would change state to a gas really quickly at high speed and altitude.
Some people are suggesting it could be to upset the sensors in the piston engine, the drone is through the cloud so fast that it is pulling in fresh air almost immediately, also, the MQ-( does not use a Piston engine, it is a TurboProp, which is a gas turbine engine.. basically.. it IS a jet engine but instead of producing thrust, it uses the rotation of the turbines to drive the propellor, allowing for a much smaller engine to drive a prop, which means the drone is more efficient an capable of flying at slow speeds, such as those speeds best for taking pictures/video. Pouring some jet fuel into a jet engine isn't gonna do much.
It is possible that they were trying to block communication, by disrupting the antenna direction, this is why the feed cuts out when the drone is buffeted, the antennas need a perfect line of sight. So perhaps the fuel cloud can disrupt communication just long enough to cause it to crash.
However.. I think it was more of a visual warning, saying these pilots are acting all kinds of crazy, and we best get out of here before we lose a $30 million dollar drone..

"So perhaps the fuel cloud can disrupt communication", this is unlikely, this would have no more effect than rain.


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How is the F-22's nacelle/engine bay ventilated?
How is the aircraft skin around the engine bay in the F-22 cooled?
As in the F-35, there are scoops below the wings, which provide ram air around the engine bay, but you don't see any scoop around in the F-22 for the same purpose.

Location of F-35 scoops according to codeonemagazine.com

As in F 35 there are scoop below the wings which provide ram air around the engine bay, but you don't see any scoop around in the F 22 for the same  purpose

The exact details of the F22 are classified. But the more likely scenario is the engine has a cold bypass air gap between the afterburner inner wall and its exterior shroud. From all the close-up shot of its nozzle, the thrust vectoring nozzle and the emergency arrestment hook fairing is lined with some sort of refractory materials and need no special cooling and the serrated nozzle fairings on either side of the nozzle seem airtight and have no place for a cooling air stream outlet. Plus, we see no scoop on the front of the aircraft as an inlet for engine cooling, and the scoop, should it exist, must be quite large. I say this is the most likely scenario because it's the least heavy and complex solution to engine cooling. And let us do not forget small cavities with a diameter in the range of millimeters is the most effective radar wave reflector/scatterer.
A less likely scenario is the engine is cooled by fuel, just like the avionics, the radar and the AC for the pilot. The fuel is then cooled first by the sheer amount of wing area as a radiator and second by a pair of hidden air-cooled fuel cooler. (I won't tell you where :))

It's not about the engine cooling, if you want the aircraft to be more stealthy the area around engine also be cooled. There are 6 diamond shaped opening in the rear bottom fuselage of the aircraft are they meant to provide air into engine bay

@AshishBhatt can you include the link to your claim in your original question?

@AshishBhatt I'm just speaking from my memory. I don't think there is an authoritative illustration of the underside of the Raptor. But from what I know that seems to be outlets rather than inlets and they seem to be the outlets of the Primary Heat Exchanger, used to cool down bleed air to provide air to pilot.

https://www.pinterest.com/pin/524528687823913529/  this link shows a cutaway of F 22 clearly the heat exchanger inlet and outlet are on the top, APU inlet and outlet is also on the top, ECS inlet and outlet are towards the front.

https://goo.gl/images/vEVwFk In this link if you see there are 6 openings on the bottom are they meant for bay cooling?

@AshishBhatt No, I'm sure they are not inlet of any kind.

@AshishBhatt i've seen this illustration multiple times, the heat exchanger is the outlet for air-cooled fuel cooler. the f22 uses fuel as the final liquid cooling medium  which is unlike any other fighter aircraft, so it needs a pair of massive fuel cooler unlike any other fighter aircraft.

Yes @AshishBhatt Meatball's answer (although I can't be sure in any case) is at least very convincing. Generally, any cooling that would be relegated to inlets and ducts on conventional aircraft are moved to interior bleed pipes and whatnot in stealth aircraft in order to minimize breaks in the profile. Some of my uni classes go into alternative solutions to issues that were traditionally solved with r-profile detrimental solutions, and most of them delegate air movement to bleed stolen from air ingested in the main, stealthy inlets.


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Can the fly-by-wire system of a modern airliner handle an unstable condition?
According to this answer airliners/large passenger aircraft such as an A320 are designed such that they are longitudinally, statically stable under normal flying conditions.
What if, for some reason, the c.g. would suddenly shift just aft of the neutral point? Could the FBW system of (for example) an Airbus handle this, just like the FBW of a fighter jet which is unstable by design?
Of course I'm not talking about a situation where the c.g. ends up so far back that the aircraft enters a deep stall, but a condition just "after" neutral stability, where the aircraft has just become "slightly unstable".

If the reaction time of the FCS is significantly shorter than the short period mode of the aircraft, then yes.

Which will probably be the case, right? Or is it not possible to generally assume this?

The very first implementation of the fly by wire system in a real air craft was the X31 which was an experimental fighter jet. https://en.wikipedia.org/wiki/Rockwell-MBB_X-31 Since fighters need agility they are aerodynamically unstable to enable high turn and gear rates.

Yes, even when the aircraft is in the positive static stability region, there are constant disturbances that would change the aircraft pitch attitude and AOA away from the desired/programmed pitch. In the F-4 we had attitude augmentation systems in all three axis, pitch, roll, and yaw, which, when they detected small deviations, would input flight controls to correct for them. They were effectively eliminated attitude wandering.. FBW systems would work similarly, and be even more effective, in any static stability region, regardless of the dynamic stability characteristics of the aircraft

Usually we call this "robustness", basically the ability for a control system to control something out of spec. Like what if your target is over weight, under power, or unbalanced, or more or less stable than expected. Some system are designed to be super robust, some are less so. It all depends on all kinds of tradeoffs.

So everyone before answering this question, please note this question is not about controlling unstable targets, but about targets with unpredictable and even unmeasurable changing dynamics, and focus the answer on the adaptive and robust side of the control system.

The best place to try it would be a full flight simulator. I've only tested one aircraft with static instability in a home sim with decent physics and it was a large passenger aircraft and I was able to control it manually after some practice. So my prediction would be as long as the FBW doesn't create an oscillation on pitch or pitch trim it should help the stability.

A slightly-unstable airplane can be flown even without FBW (example: Wright Flyer).

Yes it can - to an extent, depending on:

amount of CG shift;
airspeed;
aircraft weight (actually Moment of Inertia);
maximum deflection rate of elevators/ailerons.

If the airframe has aerodynamic static stability, a disturbance in Angle of Attack Creates aerodynamic forces that return the aircraft attitude back to neutral. If the Centre of Gravity shifts aft of the neutral point, the airframe becomes statically unstable - the aerodynamic forces now want to amplify the disturbance. The rate gyro's in the fly-by-wire system sense that the attitude deviates from the commanded attitude, and deflect the elevators/ailerons such that the aircraft is brought back.
Image source
Amount of CG shift
The ability of the Flight Control System to correct for unstable equilibrium depends on the degree of instability: the curvature of the right cup in the picture above.
Airspeed
Deflection of the elevators and ailerons generates aerodynamic moments proportional to $V^2$. At high airspeeds, a small deflection results in a large moment about the CoG - the critical situation is low airspeed, which mostly occur close to the ground. At low airspeeds large deflections are required.
Aircraft weight
If the aircraft is at a high gross weight, and if this weight is distributed away from the CoG, larger control deflections are required to correct disturbances in AoA. A heavier bowling ball in above picture makes for higher required forces.
Maximum deflection rate of elevators/ailerons
The frequency response of the flight control systems is a function of how fast the control surfaces can be brought to the required position, and this is a function of hydraulic fluid supply rate: the added capacity of all the operational hydraulic pumps.
The F-16 is designed to rapidly correct for any disturbance from the commanded attitude, in any flight situation. All systems are dimensioned such, that artificial stability can still be provided at the lowest airspeed and the highest MOI that the aircraft can encounter. The A320 is not designed with a flight critical FBW system - if all systems fail, the aircraft returns to a state with aerodynamic static stability. If the FBW system fails in an F-16, the pilot must eject.

"The A320 is not designed with a flight critical FBW system - if all systems fail, the aircraft returns to a state with aerodynamic static stability." However, what happens if an unstable condition occurs due to a shift in c.g. at the same time as a failure of the FBW system? Could the pilot then still hand-fly it?

That's a gradual thing as well, depends on the degree of instability. A pilot can provide active corrective inputs if the instability is not too rapid and violent, like the instability in a hovering helicopter which has a typical time period of tens of seconds. It takes getting used to though, and it would be tiring to do it for longer periods.

The word handle here is quite loaded. I think your question is whether a commercial transport category FBW aircraft capable of maintaining level flight for statically unstable CG. A better interpretation is, whether a pilot can safely fly the said aircraft if the CG is unstable, or nearly unstable (aft of the published CG envelope)? The answers to all of them, I would argue, are no.
First of all, to stabilize a statically unstable aircraft an AOA feedback is required in the pitch loop (see B.L. Stevens and F. Lewis, Aircraft Control and Simulation). For the commercial FBW transport category aircraft I've worked with, only pitch rate and pitch angle are fed back. AOA and other measurements are used for gain scheduling, but not directly in the pitch loop. I suspect this is the case for all commercial FBW. Pitch rate, with gain scheduling, is usually sufficient to increase the handling quality for relaxed stability configurations (static margin < 5%). I suspect a lot of this has to do with failure tolerance: if your AOA measurement is miscompared or disabled, the plane must keep flying. So pitch unstable aircraft is a no-go for Part 25.
Second, even if the CG is stable, but sufficiently aft of the CG envelope, the FBW would likely not be tuned to achieve the necessary handling quality for safe flight. The longitudinal modes would not be sufficiently damped, and you are more prone to control sensitivity. The situation worsens as you approach neutral stability.

But the A320 also measures the accelerations, otherwise how could you fly a C* law? It also has and angle of attack protection, though I don't think it actually uses the AOA to fly it (have another question ongoing on that) - I think it is a pitch and speed control loop (with dv/dt) as well...

I would like to think the C* as a tracking command, and outside of the stability augmentation (SAS). However, without understanding the A320 laws in depth, I don't want to surmise its feedback and its effect on statically unstable configurations.

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Are there any autopilots that are capable of landing a damaged aircraft better than any human?
How close are we to commercial aircraft with autopilots that can, as safely as theoretically possible, land an aircraft with damaged, missing, or otherwise inoperative control surfaces; or altered aerodynamics (e.g. ice on the wings), missing or misleading avionics (e.g., frozen pitot tubes), etc.?
Remember the Gimli Glider? An autopilot that could do that would be a good start.
And the computer in Space Shuttle Columbia made a valiant effort to keep the nose pointed in the right direction even as the aircraft was disintegrating.

Generally the autopilot functions rely on sensors and actuators/control surface intact, and cease to function when this is not the case. When a pitot is frozen, there are inconsistencies in pressure data (with redundant systems) and automation hand over control to the crew. The scenario of Gimli glider can't be entirely delt by current automation (starting with a first problem: the airfield was decommissioned and not in FMS database).

To expect a computer to analyse context and adapt no new situations is the same to expect to a human to react faster and with more precision than a computer. We are better than computers in some areas and computers are better than us in some aspects, let's use computers to suplement our abilities and surpass ours limitations. Trying to use it to fully replace us is foolish out of scifi.

This is really a two step job: first is to recover control authority with alternative controls, then second is to evaluate the result of the recovery and land the aircraft within the limits. Computer is good at the first but humans is better at the second, currently. Like if you lose the vertical stabilizer and rudder completely, it's almost impossible for a human pilot to maintain stability and control with differential throttle and air braking, but computer can do it OK. After that is handled, the pilot can just land the plane "normally".

Reconfigurable flight control systems have been a topic of research in military aircraft for decades. The science is existing and mature, but not an easy one: damage cannot be predefined, and therefore cannot be explicitly programmed in.
Classical re-configurable flight control systems attempt to re-program on the fly: the transfer function between input and expected aircraft response is constantly monitored, and if actual response differs from expected response, the gains in the loop are reset on the fly in order to get closer to a stable, matching response with neutral trim.
From this paper, which is almost 20 years old:

The goal of control reconfiguration is to maintain handling qualities in the presence of a large universe of damage and failure modes. In contrast to robust control design methods of the past, the emphasis in control reconfiguration involves a combination of on-line parameter identification control redesign and/or adaptation for a degraded mode of flight.

Neural networks are now being implemented in reconfigurable flight control systems. These work as the synapses in our brain: there are a lot of possible connections, which are defined in our childhood as a result of positive and negative experiences. The system is self learning, from a multitude of outcomes that are labeled either "desirable" or "not desirable". Neural networking is what makes Google Translate possible; translating a language by programming is quite difficult and often less accurate because of all of the different inflections and double meanings in a language.
So in military aircraft an auto-damage-pilot is very well possible. Civil airliners are a different matter, not because of technology but because of certification. Safety is of the highest importance, and how can we prove that our re-configuration is actually helpful?

Neural networks are not needed. There are controllers that are damage-resistant and do not use them. And they will be employed on civilian aircraft.

@Koyovis How to prove the AI is safer than human pilots in the real world? Whenever an aircraft experiences a failure that could (or did) bring down the plane, model it in a simulator and test that scenario on the AI and on human pilots.

Yes a simulator helps - problem is that normal training simulators have a data pack that stays within normal flight conditions, AoA about +/- 20 deg and angle of sideslip about 30 deg. How would you get data of a damaged aircraft to validate the simulator against? And how to implement higher AoA and sideslip data? CFD, wind tunnel data etc is a possibility, but it is still computed data, not real life flight data and that is what the authorities want to see.

Here is another more recent technical paper.

@jjack  getting the physics right in a simulation is very complicated. So we have to simplify, or use data tables from real life. With damaged aircraft you can't get any data table to help you, and it will introduce lots of side effects that are more complex to model.

@Koyovis: Some types of damage (total loss of hydraulics, vertical stabiliser falling off completely, etc.) can be predefined for their worst-case states; if, instead, an aircraft suffered a less-total instance of one of these (for example, only five out of six hydraulic systems failing, or only half of the vertical stabiliser falling off), this would merely add an extra margin of safety.

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Which instruments are assigned to Pilot Flying and Pilot Not Flying in the various flight stages?
Is there a document which highlights the responsibilities of the Pilot Flying and the Pilot Not Flying? Specifically, I want to find out what instruments are assigned to whom in all the stages of flight.
I haven't had any luck in finding any documents which highlight these points.
So far I have read through the FAA handbooks but to no avail.
For example:

During takeoff, PF focuses on X and Y but PM monitors X, Y, A and Z
Then during descent, PF focuses on C and F but PM monitors C, F and A

Are there any resources which I have failed to look at? Does any document explain this?

Welcome to aviation.SE! This might also depend on a specific airline's operating procedures

The general duties of Pilot Flying (PF) and Pilot Monitoring (PM), also called Pilot Not Flying (PNF), are e.g. outlined in the following section of the Boeing 737 NG FCOMv1 (NP.11.2 Normal Procedures - Introduction, emphasis mine):

Crew Duties
Preflight and postflight crew duties are divided between the captain and first
  officer. Phase of flight duties are divided between the Pilot Flying (PF) and the
  Pilot Monitoring (PM.)
Each crewmember is responsible for moving the controls and switches in their
  area of responsibility:

the phase of flight areas of responsibility for both normal and non-normal
  procedures are shown in the Area of Responsibility illustrations in this
  section. Typical panel locations are shown
the preflight and postflight areas of responsibility are defined by the
  "Preflight Procedure - Captain" and "Preflight Procedure - First Officer."

The captain may direct actions outside of the crewmember’s area of responsibility.
The general PF phase of flight responsibilities are:

taxiing
flight path and airspeed control
airplane configuration
navigation.

The general PM phase of flight responsibilities are:

checklist reading
communications
tasks asked for by the PF
monitoring taxiing, flight path, airspeed, airplane configuration and
  navigation.

PF and PM duties may change during a flight. For example, the captain could be
  the PF during taxi but be the PM during takeoff through landing.

So monitoring instruments is primarily the responsibility of the Pilot Monitoring (hence the name). But of course the pilot flying will also monitor instruments like e.g. airspeed indicator when hand flying.
The exact duties for each phase of flight are also outlined in the FCOM, but they are of course aircraft specific. The following is part of the 737 takeoff procedure:

If you are interested in more details, I suggest you try to find an FCOM (Flight Crew Operating Manual) for the aircraft you are interested in online. It should have detailed lists of responisibilities for each phase of flight.


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How would a hole in the side of a plane affect its flight?
Let's say an explosion blew a hole in a plane's fuselage without ripping the aircraft apart or damaging the engine and wings. How would the hole affect its flight?

There's likely be some increase in drag. Beyond that, the effects will depend on the size and location of the hole. Modern airliners can fly with remarkably large holes. See Aloha 243.

Related:. https://aviation.stackexchange.com/questions/15813/is-it-possible-for-an-airliner-to-safely-fly-with-doors-open

Some aircraft can lose an entire wing and still land.

Obviously, there is no safe way to do this.  OP should be aware that, with many planes, the fuselage skin is load bearing, along with the frame.  A little like saying "can I blow a hole in the side of my house to put in a window".  If what you are destroying reduces load bearing safety margins, no, it is not a good idea.

Most planes fly best without any extra holes, of course! The specific effect of a hole is modifying local airflow, increasing turbulence, and increasing the aircraft's wetted area on the side of the breach.
As long as there isn't a matching hole in the back, however, the airflow inside separates (similar to a pickup truck), so most of the airflow is still normal around the fuselage. The turbulence will also disturb flow over the wing root, if the hole is located forward of the wing, slightly reducing lift on that side. All in all, range will be reduced and more control inputs will be required.
Other than that, a hole in the side does not prevent a plane from operating normally. One can think of a conventional aircraft as the wing, pushed forward by the engine, and stabilized by the tail. The fuselage just connects them together and houses the payload (and controls, but we're not thinking about that at the moment).
Most fuselages are streamlined for minimum drag, but a plane can fly with a non-streamlined fuselage as well, just not as fast and not as far. Fixed-wing gunships, for instance, fly operationally with large holes in their side for the armament.

And then there's this puppy: NASA/DLR's SOFIA, a 747 with a giant hole in the back for a telescope: https://youtu.be/g5z6fZKOtP4


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How can I calculate turbofan efficiency as a function of turbine inlet temperature?
I know that increasing the turbine inlet temperature (TIT) increases the overall efficiency of the jet engine. So I wanted to find the overall efficiency as a function of TIT. I used the equations from this website. But the efficiency seems to go down as I increase the TIT. I have attached the pictures of the functions I used to get the graph.
I assumed a compressor pressure ratio of 30, a fan pressure ratio of 1.5 and a bypass ratio of 8.
stages of the cycle the way I defined it as
1-2 is inlet
2-3 is compressor
3-4 is combustor
4-5 is a turbine
5-6 is afterburner(I didn't use it so 5 and 6 are same)
6-7 is the nozzle
I have also found another picture from this website that showed the similar result with efficiency going down as TIT is increased

Can someone please tell me what I did wrong? Or if you can provide a similar graph with similar assumptions that would be great.

First, you should state whether you are designing an engine (increasing the TIT) or if you are doing an off design calculation for an existing engine.
In the case of design we gas turbine performance engineers call this design point analysis, increasing the TIT indeed would increase the efficiency, but this would require an increase of the pressure (OPR, Overall Pressure Ratio) as the pressure ratio is a parameter in the thermodynamic efficiency equation. A rise in compressor exit temperature by the increased pressure ratio requires raising the TIT to be able to add power/heat to the cycle. See e.g. the following example:

When increasing the pressure, the exit temperature of the high spool speed compressor also increase and results in an even higher amount of cooling flow required to cool the turbine. Using simple formulae is possible, but not preferred, specific software exists to do this for you.
If you are doing an off design analysis, so basically you have a frozen design and want to see the effects of TIT on efficiency, this can only be done with a gas turbine simulation program. To do this you would require the off design characteristics of the turbomachinery components, which are highly classified data.
Looking at the formulae you posted, I assume that you are looking at various designs. Please download a free copy of a gas turbine performance simulation tool and it will tell you the answer in minutes. E.g. this tool or this tool are free to use. There are probably much more tools.


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Can aircraft stall at low speed during final landing?
Is it possible for an aircraft to stall when it turns multiple times before landing and its airspeed is below 200mph? And if it does, is there any way to recover at such low height and speed? I always get nervous looking out the window and feeling that 1.5g in my seat during landing.

Have you read the answers to this question: How does stall depend on angle of attack but not speed??

You need to remember that the pilots are the first occupants to arrive at the crash. They are well trained and experienced in control of the aircraft. Therefore they will not put themselves in the position of arriving first.

When you learn to fly one of the first things that you learn is that recovering from a stall at low altitude is rarely successful. So we go up to at least 1,500 Above Ground Level (I like to go even higher) and we practice putting the airplane into a position just before it stalls—the stall warning light goes on and the wing begins to buffet. Then we use appropriate power, ailerons, and rudder to recover from the stall. Every pilot is tested on this before they receive their pilot’s license. Commercial pilots and air transport pilots are tested again with higher standards than private pilots. The idea is that you know the conditions that result in stalls and avoid getting into that situation. If for some reason you do stall the airplane, your training should kick in and you should be able to recover.
Unfortunately, many general aviation pilots get distracted on final approach and there are frequent stall-spin accidents in light airplanes. They are extremely rare in the airlines. The only one that comes to mind is the Colgan Air accident in 2009 where the plane iced up and the pilot flying ignored the stall warnings and pulled up on the stick instead of pushing the nose down and adding power.
We are taught that a stall can occur at any airspeed and any pitch if the airplane exceeds the critical angle of attack. There are lots of ways you can do that but the two most common are banking and icing up the wing. The Colgan Air crash is an example of the later. When the wings are iced up they do not provide as much lift as when they are clean. In order to keep the plane level you need to increase the pitch. This increases the angle of attack and at some point the wing exceeds the critical angle of attack and the plane stalls.
I am not aware of any recent airline accidents related to stalls while banking but there are plenty in light aircraft. An unfortunate series of events resulted in a stall spin accident in a Cirrus at Houston’s Hobby airport in 2016.

In the example you gave in your post of a 1.5 g load factor the stall speed increase is only going to be a few percent. So if you were in a Boeing 737 with a stall speed in the landing configuration of ~125 knots, the stall speed in the turn would be a few percent more—say 130 knots. The pilots know this information and will keep the angle of bank and airspeed within limits.
One of the maneuvers that we must master to get our Commercial license is steep turns of 50°. My Cherokee has a stall speed of 55 MPH in the approach configuration. The stall speed at a 50° bank angle is approximately 1.28 times 55 = 70 MPH. For me to successfully do a steep turn, I need to enter the maneuver at an airspeed of greater than 70 MPH.
Another maneuver that we must master is an accelerated stall. In this case we start out at our approach airspeed (in my Cherokee it is 80 MPH) and we bank to 45° and then pull up on the yoke. As you can guess from the discussion above, the airplane almost immediately stalls and then we recover from the stall. The reason we practice this maneuver is so that we don’t do this when coming in for a landing.
What causes pilots trouble is forgetting that your stall speed increases dramatically in a steep turn. Suppose you are turning to line up with the runway and you underestimate the wind and it blows you past the runway centerline. The natural reaction would be to steepen the turn. But when you steepen the turn you change the vertical component of lift so you start to sink toward the ground. If you pull up to get away from the ground you lose airspeed as well. So now you’ve increased the angle of attack past the critical angle of attack. The airplane stalls and if you weren’t coordinated with the rudder (which is often the case) the airplane enters a spin. This close to the ground they are rarely recoverable.
Stalls occur because the airplane exceeds the critical angle of attack. In most normal flying the angle of attack is directly related to the pitch and bank angles and the airspeed. We are taught how to avoid stalls in our primary training and when we get our refresher training every two years. We still manage to mess it up. Airline pilots receive much more training than we do and it is reflected in the accident statistics.

What aircraft did you train in that has a stall warning light? Did you mean AoA indicator?

@RonBeyer My Cherokee has a stall warning light in the middle of the panel. Other aircraft that I have flown have a stall warning buzzer.

Old Pipers tend to use a light

Every aircraft has a stall speed, usually landing above stall speed. When an aircraft stalls, this means the aircraft no longer generates lift to keep it in the air and falls down from lost lift. This doesn't occur in everyday landings. If an aircraft were to stall, the pilots made gross errors in flight landing configurations, unexpected large wind gusts were encountered, a microburst occurred during the landing phase or the aircraft systems failed. At approach or landing speeds, if a stall occurs, aircraft may not have enough altitude to recover where dropping the nose to dive will increase airspeed above stall speed to regain flight control. Either the airplane crashes or lands with damage to landing gear or more severe damage occurs.  Virtually every commercial aircraft never flies at or near stall speeds when landing from training and familiarity with each airplane. When done correctly, pilots carefully manage airspeeds during every phase of flight as professionals.


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How should I log time when I fly from one airport to another and back again without landing?
How should you log flight time when flying from airport A to airport B and return to airport A without landing at airport B?

Welcome to aviation.SE! Please tell us which country or regulations you're asking about: different countries have different rules.

The time from when you started the engine to when you stopped the engine.  It doesn't matter where you went, except for where you specifically need to record the trip details for training requirements purposes, or for your own historical record.  So you'd log the overall trip time, startup to shutdown, adding something like, "CC YYY to XXX back to YYY - no landing at XXX" in the Remarks.

Just to clarify. Nothing goes in the cross-country column since there wasn’t a landing 50+ miles from where you started.

@JScarry I guess you're talking about FAA regulations? 61.1 has seven different definitions of cross-country time and not all of them require 50NM or a landing :-)

@Pondlife The question seems like it is coming from a student pilot, so that’s why I added the 50+ miles since thats what they need for FAA pilot certificates. Since there was no landing at all, it doesn’t count as cross country. Some military pilots can count flights without a landing as a cross country toward a rating, but I don’t believe anyone else can.

or as was done during my training, just log "XXXX local 1 hour" and never even mention where we went.


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Is it okay to divert to another airport in IFR lost comms situations?
In the event of lost communications, FAR 91.185 allows pilots to divert to a nearby airport if they encounter VMC conditions en route to their destination. But what if IMC conditions persist? Are they legally allowed to divert to another airport, which is not designated as the alternate airport in the flight plan, and shoot an approach to land? Doesn't that conflict with the regulation requiring pilots to follow "last assigned, last vectored, EFC route, route in the flight plan" in order?
I ask this question because I came across this phrase in my study material called Everything Explained (p.82): "Do not continue on to your destination if there is a suitable airport for you to land at, unless you're just minutes from your original destination."

A relevant column just appeared in Air Facts 'What To Do When The Panel Goes Dark' where they talk about using all the resources available to you—in and out of the cockpit. http://airfactsjournal.com/2017/06/panel-goes-dark/

Short answer: you're expected to follow the rules you mentioned unless it's an emergency.
The regulations are in 91.185 and they say (among other things):

Unless otherwise authorized by ATC, each pilot who has two-way radio
  communications failure when operating under IFR shall comply with the
  rules of this section.
  [...]
  If the failure occurs in IFR
  conditions, or if paragraph (b) of this section cannot be complied
  with [i.e. the aircraft doesn't enter VMC], each pilot shall continue the
  flight according to the following: [route, altitude, clearance limit
  rules]

As written, that seems clear: if you aren't in VMC and don't enter VMC, you "shall" continue your flight according to the usual rules. The AIM 6-4-1(c) says the same thing:

In the event of two-way radio communications failure, ATC service will
  be provided on the basis that the pilot is operating in accordance
  with 14 CFR Section 91.185. A pilot experiencing two-way
  communications failure should (unless emergency authority is
  exercised) comply with 14 CFR Section 91.185

The whole point of 91.185 is that you don't want to fly around literally blind in IMC with no way to know what other aircraft are out there. And especially in busy airspace, ATC and other pilots certainly don't want an aircraft they can't see behaving unpredictably. If the comms failure also took out your transponder, for example, then ATC could only track you on primary radar, which is quite limited.
Having said that, the AIM is clear that if the PIC decides that the situation is an emergency, then he can do whatever seems best. Comms failure by itself isn't an emergency (see the AIM 6-4-1(b)), but every situation is different and if other equipment fails or you have other safety issues to deal with then diverting might be the best decision. There are just too many possible scenarios to have a strict yes/no rule.

AIM 6−4−1. Two-way Radio Communications Failure
a. It is virtually impossible to provide regulations and procedures applicable to all possible situations associated with two-way radio communications failure. During two-way radio communications failure, when confronted by a situation not covered in the regulation, pilots are expected to exercise good judgment in whatever action they elect to take. Should the situation so dictate they should not be reluctant to use the emergency action contained in 14 CFR Section 91.3(b).

That's it. If I lost all comms & datalink said both mega airport A (destination) and mega airport B (alternate) were near mins (read, congested, possible holding, maybe below mins whenI arrive), but medium-size airport C is well above mins and nearby, I'd feel good about diverting there. Probably try to pass intentions via that datalink if possible -- works better w internet/ACARS than with ADSB, obviously. Might try cellphone to FSS once low enough to get signal.

The whole point of setting the regs regarding lost com is to enable ATC to anticipate the pilot's course of action in that situation, isn't it? But if a pilot diverts to an airport that's not in the flight plan, would that "surprise" ATCs?

@lemonincider In the modern ATC system you are most likely being tracked by ATC on radar so ATC can figure out what you are going to do. If you are 1/2 hour into a 4 hour flight and pass near a Class E or D airport with an approach, good sense dictates that you land for you own safety and the safety of others. Maybe what cased the comm failure is only the beginning of your problems. If you have comm failure, how likely is it that you also nav failure? Call clearance delivery 888-766-8267 and get a phone number for the  ARTCC in your area and communicate by phone—not in the regs.

I agree with all answers given, but will add this:  If IMC and deviating from Asssigned, Vectored, Expected or Filed, don't just squawk 7600, roll it to 7700 to get their attention and let them know you maybe aren't going to be as predictable as they might think...
Also, I was always taught to broadcast intentions in the blind just in case they can hear you, but you can't hear them.  You may not fully understand the exact failure mode causing you to be lost comm.  I once experienced an ICS/ headset failure where I couldn't hear a thing, but the controllers and my crew could hear me transmitting just fine.


text:
What is wrong with a high-speed landing for an ultralight or GA aircraft?
Is it okay to land an ultralight or a general aviation plane at a speed higher than the recommended landing speed for such plane? Will there be any damage if the descent rate is very low?

Related. Higher speed landings are common in training, e.g. to demonstrate the effects of a tailwind, or to simulate a flaps failure.

You can, in theory, fly a faster landing speed than the default Vref of 1.3*Vso.  But it’s wasteful in the round out and potentially very dangerous on short field landings.
When an airplane lands, the pilots needs to enter the round out with only enough energy to arrest the descent and establish the airplane in the correct attitude for touchdown approx 1-2 ft above the runway.  The airplane should be then quickly be exhausted of energy and set down on its landing gear, ideally within the first 1/3 of the runway length.  Excess speed here combined with the dramatic reduction in induced drag due to ground effect will cause the aircraft to ‘float’ down the runway until it slows enough and touches down.  Attempting to force the airplane onto the runway with forward elevator pressure is hazardous as the PA-28 in the video below shows.  A fundamental rule of flying airplanes is never force an airplane to land if it does not want to.  Go around and try again.

A graphic example of why floating is risky on short field landings comes from this one of a Piper Aztec attempting to land at Gustaf III on St Bart's.  Pilot was too high and 10-15 kts too fast.  The float used up critical runway the pilot needed for braking and by the end was unable to stop with the runway available.

Couldn't the Piper just go around (after seeing that it floated way too much)? Or maybe it didn't had enough power to?

@gsamaras - I suspect that the pilot just wanted to get the plane on the ground. And y'know what? HE DID!

@gsamaras Yes, they should have. This is a perfect example of most of the Classic Five Hazardous Attitudes.

Yes the PA-28 should have made a go around

Maybe it should be a question on its own, but at the St Barts airport (2nd video), why don't planes approach from the other direction. Looks safer to me (easier to use the whole runway), but what do I know?

The can fly the Rwy 28 approach but the winds were favoring Rwy 10 that day.

Keeping too much speed in an approach in any airplane can be risky depending on how much runway you have as it all has to bleed off before you can stop. It's a great way to end up in a hedge. Extra airspeed means you'll either float in ground effect for a very long time, or if you somehow manage to get the wheels down you won't get much braking action as the wheels won't have much weight on them on a tricycle configuration, in a tailwheel configuration you will have braking action but you'll have to be gentle or you'll nose in. In fact, trying to hold down an airplane that is far over rotation speed is tricky, and it can all go wrong. There's no good reason you'd want to try this unless you had some sort of condition in the airplane that necessitated it.
Landing fast isn't good for your tires, the extra ground speed will cause more wear on initial contact as the wheel has to spin up that much faster. Your brakes could overheat or at least would wear out faster as well. Other than that I can't think of any particular damage or wear.

"Other than that I can't think of any particular damage" when thinking of ULF you have to keep in mind that they usually have low safety factors and pushing your gears to the ground at high spead may cause damage to them especially if the runway is gras or similar.

Sure, @TimothyTruckle, but that's no different from the risk of any other landing if it's mishandled. You have a greater risk of a hard landing if you land fast, but landing fast doesn't automatically mean you land hard.

Depends.
With tricycle gear airplanes the limitation is mainly pitch attitude in the landing flare so you don't come down nosewheel first, which is very bad on the nosewheel and ultimately on the airplane.  So you need to be slow enough that the mains will touch first with landing flap.  But beyond that it's just wear and tear on the tires and brakes.
With taildraggers it's a different story.  If you do wheel landings (tail high touchdown), which is my preference, you can, in theory, fly the thing on to the runway at any speed you want within reason, limited only by what the tires can take and possibly propeller clearance if going too fast.  In fact a normal wheel landing is done quite a bit faster than a 3 point, touching down in a relatively level attitude maybe 10 mph faster than a full stall 3 point landing.  You actually fly the airplane down to about 1 or 2 feet above the surface and push the stick to plant the mains when doing a wheelie.

I don't think there's anything inherently dangerous about landing a light aircraft faster than the prescribed speed, however you're unlikely to be very successful. The most likely outcome is just a continuous float down the runway until the airspeed drops off anyway.
Landing an aircraft is more or less stalling the wings in a controlled manner, very close to the ground. This causes the wings to no longer be taking the weight of the aircraft, and gives you the ability to use wheel brakes.

Someone told me once:
When landing a plane the stall horn should sound just as you touch the ground, and this is basically how you land: Gradually reducing lift from your wings to the point where the plane's weight fully rests on it's wheels. If you go too fast there might be too much lift to touch down!
Landing at a low descent rate won't damage the airplane at all, but the higher ground speed required to do so might, as others have pointed out.
While you can attempt to land at high speeds, the lift still produced by the wings can make it hard to stay on the ground and keep the plane intact at the same time. Gliders, which are obviously very light, use airbrakes to reduce lift/increase drag drastically when descending and landing.

ASK 21 glider with deployed speedbrakes. Source: Wikimedia Commons
For planes without any airbrakes a fast approach most likely results in a long float above ground due to the lift, or a long roll on the ground, for which you may not have the room on short airfields, as mentioned by others already.

I have a real life example of a high-speed approach incident
(Recollection from memory - might not be 100% accurate)
A couple years ago a pilot flying our club-owned ultralight arrived back at the airfield. They wanted to get it down quickly, my guess is because of gusting winds, but unfortunately their airspeed was way too high for landing. I do not know whether this was due to the winds or pilot error.
Instead of going around, or using the generous runway length to slow down further, they wanted to touch down early. The plane was still floating above ground because of it's high speed, so they pushed the nose down in an attempt to land it, which is almost instinctive, but also very wrong to do. The plane bounced off the nose gear two or three times before it collapsed. Prop and nose hit the ground, slowing the plane down rather quickly after that.

Aftermath of the described incident, picture mine
Nobody was hurt in the crash, but the plane was trashed. Trainees at the airfield learned a valuable lesson that day: Why you should land at appropriate speeds.

If a tricycle-geared airplane attempts to land with too much speed, the nose wheel touches first, which has all sorts of undesirable dynamic consequences.
The position of the nose wheel gives it plenty of leverage, and if it touches with any force (or the ground has any roughness), it will pitch the airplane nose-up, and the airplane has sufficient extra speed to balloon as the angle of attack increases. Once it leaves the ground, however, the aerodynamic controls reassert themselves, and the pilot is holding the elevator in the position that led to the high-speed touchdown in the first place. Unless he does the right thing at this point, which is to hold off until the speed has decreased (or go around), the nose wheel will come down again, probably harder than before (especially if the pilot tries to force the airplane down), and the cycle will repeat.
Furthermore, if the rotation of the airplane causes the main wheels to touch, this will immediately bring the nose wheel down again, hard. This can be seen in the first of Carlo Felicone's videos, where this starts happening after the second or third bounce, and after about six more increasingly violent and rapid bounces, the nose wheel collapses.
Your problems are not over if you manage to get through this phase. Now you are "wheelbarrowing", with most, or all, of the contact force being borne by the nose wheel, and any bump on the ground before you have slowed down to a correct landing speed can start the above cycle of bouncing. Furthermore, with the point of contact being well ahead of the center of gravity, you are unstable in yaw, and the slightest swing, uncorrected side-wind or drift could start a groundloop. It is basically the same reason as why taildraggers are more at risk of groundlooping, but with a longer moment arm.

I like to view landing as a critical transition period between two distinct regimes of vehicle control. The way that you handle an aircraft is incredibly different in these two regimes and many landing accidents have failing to completely transition from the flight phase to the ground-handling phase as a primary factor.
The whole idea behind the flare is to have the wheels touch the ground at nearly the same instant that the wings stall when the aircraft is in the proper landing attitude at the correct speed. And though landing incorrectly will often put you in an attitude that is detrimental to the aircraft, I would argue that its design and gear location are driven by this purpose rather than the landing attitude being driven by the aircraft/gear design.
One operation where this is quite visibly apparent is in landings with very high crosswinds. Since the method for handling an aircraft in a crosswind are particularly different in the two regimes, the maneuvers to transition between the regimes at landing are necessarily very pronounced. Sometimes you will even feel like you have to force the aircraft into the transition in extreme conditions.
If you have experience riding motorcycles, I find it quite similar to the difference between low-speed handling where you simply turn the direction you want to go, and where because of wheel's gyroscopic procession you actually counter-steer at high speed.

authors: Bob Jarvis - Слава Україні


text:
Is it legal to fly under the Golden Gate Bridge?
I recently had the opportunity to rent a plane and fly in the San Francisco Bay Area.  While flying in the HWD/OAK/SFO area is challenging enough, when I spoke of my adventures of flying above Alcatraz and the Golden Gate Bridge, some of my fellow pilots asked me if I flew UNDER the Golden Gate Bridge and voiced their desire to do so.  As a relatively new pilot (3 months), while flying is definitely an adventure for me, flying under a suspension bridge is pushing my minimum safety limits and just does not seem lawful.
Others have told me that they have seen GA aircraft fly under the bridge. While I'm sure pilots have done it...I don't see how this can be legal.  According to sources on the internet the bridge is only suspended 270' above the bay.  It doesn't seem enough space to fly through legally.
According to 91.119
Except for takeoff or landing, no person may operate an aircraft below the following altitudes;
a. Anywhere. An altitude allowing, if power unit fails, an emergency landing without undue hazard to persons or property on the ground.
c. Over other than congested areas.  An altitude of 500' feet above the surface, except over open water or sparsely populated areas. In those areas the aircraft may not be operated closer than 500' to any person, vessel, vehicle, or structure.
To me, 270' does not meet the legal requirements set in 91.119(c)  I don't consider the SF Bay or water under the bridge to be "open".
So, is it legal to fly under a suspension bridge like the Golden Gate or Verrazano-Narrows Bridges?

While the regulations detail the same requirements, my post deals specifically with the legalities of flying under a bridge.

The regulation seems quite clear. The bridge is 270 feet above the surface of the water, and you are required to maintain a distance of 500 feet from it. Therefore, it is legal to fly under it only if you are flying at least 230 feet below the surface of the water.

As @TannerSwett says.  Even if you do consider the SF Bay "open water", the bridge itself is unquestionably a structure, which means you're not legally allowed within 500 feet of it.  If it's only 270 feet high, then you can't legally pass under it.  Double the height, however, and you'd be okay (just barely).

No, it would violate regulations unless it was to meet the needs of an emergency.
Don't forget the catch-all regulation about reckless operation; many regulations may be interpreted very subjectively, and I feel that it is safe to say that flying under the Golden Gate bridge would attract a lot of negative publicity and would ultimately be considered something that the pilot of a fixed-wing aircraft would certainly be reprimanded for.
That's not even considering whether the bridge is 500 feet high, boats that may or may not be passing under the bridge, and your question completely truncated the 91.119(b) that lists information for congested areas, which I am almost certain the Golden Gate bridge would be subjectively qualified as.

Just as a curiosity, would there be more lenience for choppers?  Or is it exactly the same?

@Fattie A local helicopter tour operator seems to do it semi-regularly, apparently under the exception in 91.119(d)(1). I'm sure one can have a good argument about the meaning of "without hazard to persons or property on the surface," but it seems to have been considered acceptable so far.

astonishing, @ZachLipton !

It’s illegal under 91.13 and any FSDO inspector who found out you did a damn fool stunt like that would yank your license in a heartbeat.

§91.13 Careless or reckless operation.
  (a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
(b) Aircraft operations other than for the purpose of air navigation. No person may operate an aircraft, other than for the purpose of air navigation, on any part of the surface of an airport used by aircraft for air commerce (including areas used by those aircraft for receiving or discharging persons or cargo), in a careless or reckless manner so as to endanger the life or property of another.

On rare occasions pilots have flown under bridges and other structures as part of demonstration flying.  They are all experienced aerobatic pilots with thousands of flight hours doing low altitude demonstration flying under their belts and have specific permits issued by the FAA in order to make these kinds of demonstration flights. THEY ALSO UNDERSTAND THAT THIS KIND OF FLYING IS INHERENTLY RISKY, HAVE CAREFULLY PLANNED THE STUNT IN ADVANCE PRIOR TO EXECUTING IT AND ARE WILLING TO ACCEPT THESE RISKS AS PART OF THE PERFORMANCE.
If you are found to have done this you will be sanctioned and potentially lose your license. If your attempt results in an accident causing loss of life you can face severe civil and criminal penalties for wrongful death and manslaughter.  If your friends are suggesting you do it, they’re idiots.  Don’t try it.

Flying under bridges also occurs in air racing, Peter Besenyei has flown under the Chain Bridge in Budapest numerous times.

You are NOT Peter Besenyei.

So when did I claim to be Peter Besenyei?

i.e. don’t get any bright ideas about that stuff.

It is not illegal, but it is a violation of FAA regulations, which could result in various civil penalties.
You quoted the violation yourself: you are not supposed to fly closer than 500 feet to any building or structure, unless engaged in specific activities, like crop dusting. The deck of the bridge is about 250 feet above the water, so you would definitely be too close.

How sure are you it is not illegal?  Criminal reckless endangerment is certainly not out of the question.  Getting into the cockpit of an airplane does not shift the paradigm of law.  Saying this is not illegal would not be unlike saying that punching a pilot in the face is not assault because threatening and intimidating flight crew is simply prohibited in the FAA aviation regulations. While it could very well go unpunished from a criminal viewpoint, you cannot make guarantees that it would.

Is it"illegal" to violate FAA regulations?  In what sense is it not "illegal" to violate FAA regulations?  Are the regs not essentially laws?  Of if not, is there not some law that says you have to follow the regs?  For example Congress passed a bill last October that directed the FAA to make very specific changes to FAR 107-- are you saying this has nothing to do with lawmaking?  (This could be a separate question for Av  Stack Exchange.)  PS these are real questions here, I'm "asking" you not "telling" you--

@quietflyer I realize that you might not be an American. In the United States there is a difference between laws and regulations. Laws are created by elected officials. Regulations are created by bureaucrats that have not been elected. In United States criminal charges can only be brought against someone who has violated a law, meaning rules created by ELECTED officials. Breaking rules created by unelected officials is not considered illegal and is called instead a "civil infraction". This is an important principle of what we call a Republic.

Thank you for the clarification.  It might be a good topic for an actual question to A.S.E.  I am a US citizen BTW.

The fact that in at least some cases, the FARs are created in response to very specific directives from Congress essentially outlining exactly what the new FARs need to be (case in point legislation passed in October 2018 outlining very specific airspace restrictions on non-commercial sUA's, eg model airplanes) is not relevant here?

I do appreciate your insight on this, I hadn't thought much about it.

Related point-- just as the regulations are not laws, the AIM is not a regulation.  The point comes up in my discussion here https://aviation.stackexchange.com/a/64231/34686

Yet in the context of say, requirements imposed on an airline passenger, some regulations surely do either function as laws or are associated with laws?  How does this work exactly?  Are the requirements imposed on the passenger explicitly written as laws and not regulations?